2001 Chevy silverado P0300 and P0430

I have a 2001 chevy silverado (110,000) that was making a bolt in spinning drum type of sound from the bellhousing area. I was told by several people it was either a cracked flexplate or or bad torque converter. Needless to say, I pulled the transmission, flexplate was fine, so I replaced the entire transmission with one of half the miles. The noise is still there (starter is possibly sticking), but when I replaced the transmission, I also did plugs, sires, and a pcv valve.

A few days later after getting up and running, the service engine light came on. Engine ran fine, no stumbling, hesitation,no transmission slippage, nothing. Couple days later i needed gas, and filled up, about 5 miles within the gas station the engine started stumbling, sometimes so bad i thought the engine was going to die. I had abuddy scan it, and came back as p0300 and p0430 bank 2. Now after resetting the gaps on the plugs, replacing the wires again, the idiot light shutoff while STILL having a hestiation and stumble. I did not clear the codes.I gave it 2 cans of sefoam, 1.5 in the tank, and the other in the vacuum line. The hesitation and stumble is not nearly as bad as before, and it is hit or miss. Sometimes it will do it and sometimes it will not. It will do it going up a hill or with trying to acclerate quickly.
Talked to a local dealer, and they said midwest gas stinks and said it was possibly a fuel pressure regulator. The gas doesn't make much since because the light was on before the fillup, but then again it didn't hesitate or stumble until after. I plan on changing the fuel filter and doing a fuel pressure check, but it is a blizzard outside, seriously.

The engine revs fines,but once under load it will stumble.
Let me know if more information is needed.
Thanks

Hey there DD, were you successful in correcting the stumbling engine?
My 2003 Chevy Suburban suddenly cut out into the stumble mode and then the DIC informed me: "Engine at half power", NK.

Prior to this error code, the engine knock sensor error code had been activated, P0327. I haven't got around to changing it, so I am presuming that it's contributing / aggrevating the engine idle and thus effecting the P0300 failure code. Consider replacing your knock sensors.